1. Field of the Invention
This invention relates to compact zoom lenses suited to photographic cameras or video cameras, and more particularly to compact zoom lenses having at least three lens units of which the first counting from front is of negative power. Still more particularly it relates to a shortening of the total length of the lens system while still permitting good optical performance to be achieved.
2. Description of the Related Art
The zoom lens comprising from front to rear the first lens unit of negative power and the second lens unit of positive power both movable for zooming, or the so-called two-groups form of zoom lens, has found its use in many photographic and video cameras, since this form is favorable to widen the maximum angular field of view.
The zoom lenses of this two-group form have been proposed in Japanese Laid-Open Patent Applications Nos. SHO 53-132360 and 56-19022.
In general, such zoom lenses are designed to the retrofocus type with the minus-plus power distribution over the entire lens system and, therefore, have excellent image angle characteristics. However, the ratio of the distance between the front vertex and the focal plane to the focal length of the entire system, or the so-called telephoto ratio, tended to become relatively high.
Another type of zoom lens comprising from front to rear a first lens unit of positive power and a second lens unit of negative power with the separation therebetween being varied to effect zooming is proposed in Japanese Laid-Open Patent Applications Nos. SHO 56-128911 and 60-48009.
In this positive power-preceding zoom lens, because of its having plus-minus power distribution over the entire system, a rules of design for the telephoto lens type can apply. For this reason, it becomes possible to reduce the telephoto ratio to a relatively low value. This feature is advantageous at easily achieving realization of a zoom lens of reduced size with a shortened total length. As for a high zoom ratio with the use of the telephoto type design in the two-group form of zoom lens, reduced size can be obtained only when the refractive power of the first lens unit is strengthened.
However, the increase in the refractive power of the first lens unit calls for a large increase in the ratio of the shift, .DELTA., of the image plane to the minute axial displacement, .delta., of the first lens unit, or the so-called responsiveness. Therefore, the structure or construction of the operating mechanism for all the movable lens units tended to become more precise.